THE BLOOD. 163 



the body, it will appear to the student, as it does to the writer, 

 that it is highly probable that there are great numbers of com- 

 pounds in the blood, either of a character unknown as yet to 

 our chemistry, or in such small quantity that they elude detec- 

 tion by our methods ; and we may add that we believe the same 

 holds for all the fluids of the body. The complexity of vital 

 processes is great beyond our comprehension. 



It must be especially borne in mind that all the pabulum 

 for every cell, however varied its needs, can be derived from 

 the blood alone ; or, as we shall show presently, strictly speak- 

 ing from the lymph, a sort of middle-man between the blood 

 and the tissues. 



The Quantity and the Distribution of the Blood.— The rela- 

 tive quantities of blood in different parts of the body have been 

 estimated to be as follows : 



Liver one fourth. 



Skeletal muscles " " 



\ Heart, lungs, large arteries, and veins. " " 

 Other structures " " 



The significance of this distribution will appear later. 



The Coagulation of the Blood.— When blood is removed 

 from its accustomed channels, it undergoes a marked chemical 

 and physical change, termed clotting or coagulation. In the 

 case of most vertebrates, almost as soon as the blood leaves the 

 vessels it begins to thicken, and gradually acquires a consistence 

 that may be compared to that of jelly, so that it can no longer 

 be poured from the containing vessel. Though some have rec- 

 ognized di£Perent stages as distinct, and named them, we think 

 that an unprejudiced observer might fail to see that there were 

 any well-marked appearances occurring invariably at the same 

 moment, or with resting stages in the process, as with the devel- 

 opment of ova. 



After coagulation has reduced the blood to a condition in 

 which it is no longer diffluent, minute drops of a thin fluid 

 gradually show themselves, exuding from the main mass, faintly 

 colored, but never red, if the vessel in which the clot has 

 formed has been kept quiet so that the red corpuscles have not 

 been disturbed ; and later it may be noticed that the main mass 

 is beginning to sink in the center (cupping) ; and in the blood 

 of certain animals, as the horse, which clots slowly, the upper 

 part of the coagulum (crassamentum) appears of a lighter 

 color, owing, as microscopic examination shows, to the relative 



